ALLOTROPY is the term applied in chemistry to the existence of the same element in various forms, each of which, though containing no extraneous substance, possesses dif ferent properties from the others. The various conditions in which a single element can be obtained are known as its allotropic modifications, and though as yet only a few elementary substances have been observed to exhibit such modifications, yet it is gener ally believed that every clement is capable of existing in several allotropic forms. Phos phorus affords an excellent illustration of this doctrine. In ordinary circumstances, and when freshly prepared, phosphorus is a pale, yellow solid, of the consistence and aspect of wax, and to some extent flexible and translucent. It requires to be placed iu a vessel with water to keep it from taking fire spontaneously. At any ordinary natural tempera ture it appears luminous, and evolves an alliaccous odor when exposed to air, owing to a slow process of combustion taking place ; and when warmed to 140° F., it bursts into. flame, and burns vividly. Common phosphorus is soluble in alcohol, ether, the fixed and volatile oils, and especially in bisulphuret of carbon, 100 parts of which, when warm, dissolve 20 parts of phosphorus. But the same clement, when dried and kept for some days, with little or no access of air, at a temperature ranging from 446° to 482" F., passes, weight for weight—without addition or subtraction of matter—into a reddish substance, which is known to chemists as amorphous phosphorus. The color of this new variety is scarlet, brownish red, or even blackish red ; and it exists as a powder or cake, which does not evolve any odor, or readily take fire, and therefore needs not to be preserved under water. When heated to 140', and even to a temperature a little short of 482°, it refuses to burn ; and, in fact, it is questionable if phosphorus in this condition will take tire at all ; though at 482°, and above, the red variety passes back again to the ordinary or yellow phosphorus, and then bursts into flame. Moreover, amorphous phosphorus is insoluble in alcohol, ether, the fixed and volatile oils, and even in bisulphuret of carbon. Probably the most striking difference between these two forms of the same substance is, that ordinary phosphorus is a deadly poison, as is too often evidenced in the death of children from sucking the ends of lucifer-matches ; whilst the red or amorphous phos phorus is not known to be poisonous at all.—Besides the two varieties already men tioned, and which are best known, there are black phosphorus, white phosphorus, and scaly phosphorus. The only manner of accounting for the difference of properties evinced by ordinary and red phosphorus, is to refer the change to an absorption of heat during the passage of the ordinary into the red variety. It is an observed fact that such absorp
tion or disappearance of heat does then take place ; whilst, when the red phosphorus is heated till it passes back to the ordinary kind, a very rapid disengagement of heat occurs.
Sulphur furnishes another example of A. In the ordinary condition of roll-sulphur, it is a pale, yellow, brittle, crystalline solid ; insipid to taste, odorless when cold, and evolving a peculiar odor when heated or rubbed. It dissolves in small quantity in tur pentine and the fixed oils, and to the extent of 35 per cent in bisulphuret of carbon. When common sulphur is heated to 232°, it fuses, and forms a thin, yellow, limpid liquid like olive-oil ; at 480° it passes into a thick, dark-brown, viscid liquid, resembling in consistence ordinary treacle ; and if, at this stage, it be poured into water, the sul phur forms itself into a thread-like mass or net-work, possessing great elasticity, like india-rubber, not at all brittle, and so soft that it can be molded by the fingers into casts and seals. Again, this elastic form of sulphur is not soluble in turpentine and the fixed oils, or even in bisulphuret of carbon. There are also other allotropic forms of sulphur.
Ogygen may be taken as a third illustration of the same doctrine. In the ordinary form in which oxygen exists in the atmosphere and elsewhere, it is a gas with no odor, no bleaching properties, and no disinfectant powers. To a certain extant it oxidizes metals, etc.; but comparatively, it may be regarded as a feeble oxidizing agent. By sev eral processes—viz., the introduction of a heated glass rod into a jar containing ordinary air and a little or the presence of clean-scraped sticks of phosphorus in a glass vessel with a confined portion of air ; or the passage of electric discharges through or round a glass tube or bottle with air—the oxygen ot the atmospheric air is transformed into an allotropic form called ozone. In the latter condition, oxygen possesses a very strong and peculiar odor, long known as the electrical odor ; has great bleaching powers, and is regarded as the agent in the air which bleaches clothes on the household bleach ing-green ; and possesses such powerful disinfecting properties, that tainted meat intro duce% into ozonized air has the disagreeable odor destroyed, and smells fresh when taken out. Ozone is doubtless the great natural agent which removes many deleteri ous gases and vapors, and destroys infectious matter floating in or diffused through the air. See OZONE.